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Monday, 14 March 2011

Inspired by the creations of my friend Soulwire, I spent a couple of hours last night coding up a simple algorithm that generates patterns from joined equilateral triangles. A random variable acts on the hypoteneuse of the triangles in 10% of cases, making the triangles shrink in size and lean, occasionally.

I've written it in Matlab for now (great for prototyping algorithms) but will probably switch to something more visually elegant (e.g. Processing / Open Frameworks) later.

Friday, 11 March 2011

Following my last post I've now got two dynamixels working on the same network with Matlab. The flexibility of these servos means that one can be used just as a position sensor. This lets you do some cool stuff, like this simple tracking controller, where one servo follows the other. This is known as a master / slave network. The moving one is the slave as it obeys the master.

To make this work you need to set the ID of each dynamixel differently. This is obvious, but without the Dynamixel Wizard from Robotis (which didn't work on my machine) I had a hard time trying to figure out how to make this happen. In the end I discovered that by using the following command with an ID of 254 you can broadcast a new ID to every servo on the network.

(calllib('dynamixel','dxl_write_byte',254,3,new_ID))

So, just connect one servo to the network and broadcast it a new ID!

The Matlab source code for the master slave demo can be downloaded here.

IMPORTANT! - You won't feel the forces from the slave servo on the master. Do not stick your finger or anything else in the way to check!

Monday, 7 March 2011

Recently I started working with a some Dynamixel RX-28 servos from Robotis. Unlike standard hobby servos, these actuators can provide position, current and velocity feedback and also have various modes of control, including compliance. They also work on a RX485 network, meaning that they can be chained together. These features make the servos a popular choice for building small humanoids, such as the Bioloid (also from Robotis).

Though a MATLAB API is available, I found the on-line examples lacking somewhat, especially when all I wanted to do was drive the servo and log its position. The following code does just that, moving the servo 90 degrees clockwise and anti-clockwise, then displaying a log of the 'present position' and 'moving' registers from the servo EEPROM.

The 'Moving' register is set to '1' when the servo is in motion. This is a bit buggy though as you need a delay in the code for it to read anything other than '1' and it doesn't exactly stop on time. Matlab isn't hard real-time so this pause seems to affect the data logging somewhat (the motion is actually really smooth, unlike what is shown above). Take the 'pause(0.01) out of the code below and you'll see what I mean

Note that you'll need to download and register the dynamixel API with Matlab before using this code.
Here are the instruction from ROBOTIS:
http://support.robotis.com/en/software/dynamixel_sdk/usb2dynamixel/windows/matlab.htm
These instructions from Agave Robotics are also useful
http://www.agaverobotics.com/products/servos/robotis/ax12/docs/Dynamixel-AX12-Matlab.pdf
Note that I had to change the 'open device' code from the examples given above. They just wouldn't work!

I hope this is useful to someone, I spent some time searching for an example like this but couldn't find one. Of course the dynamixel servos can do much more than this but this is a nice place to start.

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About Me

I am interested in Robotics, Haptics, Assistive Technology, Creative Technology, and hacking for the sake of it.
I'm a full time robotics researcher and creative technologist on the side.
When I'm not busy, I wonder what the hell has happened.
I also like to DJ and have a cat with opposable thumbs.